Unprotected metal surfaces can suffer severe corrosion when exposed to the environment. To protect metal surfaces from corrosion, the surfaces are often treated with compositions, such as zirconium or chromium compounds, as well as phosphates such as iron phosphate and zinc phosphate. Zirconium, iron phosphate and zinc phosphate provide corrosion protection by barrier action. Iron phosphate, zinc phosphate and chromate treatments are also conversion coatings. Silsesquioxanes also provide corrosion protection, but not corrosion inhibition. Each of these treatments has disadvantages such as inadequate protection of the metal surface from the environment, expense, toxicity, and not being environmentally friendly.
Prior to painting, metal surfaces need to be pretreated chemically to improve paint adhesion as well as enhanced corrosion resistance. Conventional methods such as chromate and phosphate-based pretreatments provide these properties to metal substrates, yet they also pose human safety and environmental concerns.
Silsesquioxane-based pretreatment can provide satisfactory performance on multiple metals under various paints except electrocoat paints. This is due to the fact that a freshly deposited silsesquioxane film or not fully dried silsesquioxane film is not water-rinsable. It has to be cured first by drying. If not cured, it washes off.
This curing aspect of the silsesquioxanes limits their use in many important applications, in particular, electrocoating. Electrocoating involves coating metal with a cathodic or anodic electrocoat system. Such systems are widespread and used in the automotive industry, the appliance industry and consumer electronics industry. Electrocoating paint applications require that the metal entering the tanks cannot contain any water-soluble species as that would contaminate the e-coat bath. Therefore, the metal has to be thoroughly rinsed immediately after leaving the metal pretreatment tanks. For that reason, silsesquioxanes are not used in industry as a pretreatment prior to electrocoating. The time available between metal pretreatment and e-coating on industrial lines typically is not more than two minutes.